GB2121911A - A sound-damping element having resonators - Google Patents

Abstract

The sound-damping element comprises a continuous thin plate 10 providing areas of different effective stiffness which form resonators 11, 12, 13, 14 and resonator collectives 17, having a lower resonance frequency than an individual resonator. The stiffness variations may be provided by plate support members 15, 16, reduced stiffness of areas (31, Fig. 3) of the plate by sand blasting, by corrugations (42, Fig. 4) or by pairs of plates (50.1, 50.2, Fig. 5) with interposed wires (52) or formed with depressions (61, Fig. 6a) and adhesively bonded together (Fig. 6b). A plurality of plates may be arranged in a ventilation duct (Figs. 7 to 10) or a plate may form the upper part of the trailing portion of an aircraft wing (110, Fig. 11). <IMAGE>

Description

SPECIFICATION A sound-damping element having resonators This invention relates to a sound-damping element or component having resonators consisting of a continuous thin plate connected in a force-locking manner to a latticed frame and/or provided with net-shaped stiffenings or stiffness (or rigidity) diminutions, in such a way as to provide a plurality of resonators which are capable of oscillating independently.

A sound absorber having plate resonators in general, as well as an appropriate absorber made of plastics foil in particular, is described by F.

Mechel and N. Kiesewetter in "ACUSTICA" vol.

47 (1981), pages 83 to 88. In such sound absorbers, the sound field excites a plate into bending oscillations, which are damped by internal friction and thus bring about a sound absorption. In the above-mentioned experiments described in "ACUSTICA", a plastics foil has been so deformed as to provide rectangular surface elements of a few centimetres in length and width, which elements are bounded at the edge by a kink or knee. The kink at the edge of these plates acts as a fastening and hinders the foil at this point in its free movement. The plate is therefore excited into natural oscillation by sound waves. The wave lengths of these natural oscillations are in the frequency range up to 5000 Hz considerably smaller than the wave length of the impinging air-transmitted sound waves.The amplitude of oscillation of the plate, and thus the sound-absorbing effect thereof, becomes particularly great in the case of the natural frequencies.

So that such a plate is excited by air-borne sound into natural oscillation, an incoherent pressure impingement of the plate from both sides is necessary. For this, the plate is securely clamped at the edge into a frame. The frame is provided with a rigid terminal wall, so that a cavity filled with air arises between the plate and this terminal wall. The enclosed air then acts like a simple spring on the reverse side of the plate.

Since the individual resonators, as has been mentioned, act in a sound-absorbing manner only in a relativley narrow frequency range, a large number of different plates would be necessary to damp sound waves over a wide frequency range.

Since then plates having the same resonance frequency would be relatively far apart, the effect over the entire area and the entire frequency range would be relatively slight.

The task of the invention is, therefore, to provide a sound-damping element of the abovementioned kind which acts more effectively than the known proposals, over a wide frequency range. This problem is solved by the provision of a sound-damping element having resonators consisting of a continuous thin plate connected in force-locking manner to a latticed frame and/or provided with net-shaped stiffenings or stiffness diminutions in such a way as to provide a plurality of resonators capable of oscillating independently, characterised in that the frame and/or the stiffenings or stiffness diminutions have different stiffnesses in one or more lattices or net structures of higher order, so as to provide resonator collectives having a resonance frequency which is lesser as compared with an individual resonator.

As a result of the higher order lattice or net work structures provided in accordance with the invention the result is achieved that not only do the individual plates oscillate in their resonance frequency, but also several plates oscillate together as a collective (or collective body) at a correspondingly lower resonance frequency. The individual plates are therefore effective in several frequencies, with respect to their soundabsorbing effect, so that the total number of plates for damping a predetermined frequency range can be smaller, by a multiple, than previously. Since the resonance frequency Of a resonator collective is smaller than that of an individual resonator, in addition effective decoupling of the two oscillation systems is achieved.

In the case of a single or simple higher order lattice or network structure, the effective area of the entire element is doubly utilised; in the case of a further higher order lattice or network structure, triple area utilisation is achieved. The dimensions of a collective body of resonators should preferably be smaller than half the smallest wave length Amint to be damped, of the sound transmitted by air and furthermore the individual resonators of a collective body of resonators should have different resonance frequencies.

From this it emerges that the spacing of resonators having the same resonance frequency is smaller than Am|2. Beiow this spacing limit, the identical-frequency resonators act in composite action and form a surface absorber.

With a greater spacing, on the other hand, the resonators would work as individual spot (or point) absorbers. In the same way as a surface radiator has, as compared with a spot radiator, a very much higher radiation efficiency, also a surface absorber is more effective by an order of magnitude than a spot absorber. In order, in practice, to avoid small collective dimensions, in the upper frequency range double the number or several times the number of resonators of the same frequency may be provided.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which several individual exemplified embodiments are illustrated partially and schematically. In the drawings: Fig. 1 shows an embodiment of the soundclamping element of the invention comprising collective resonator bodies (of which only one is shown in its entirety) each comprising four resonators:: Fig. 2 shows a higher order lattice or net structure comprising four resonator collectives each having four resonators; Fig. 3 is a cross-section illustrating a thin plate having resonators thereon, produced by sandblasting; Fig. 4 is a cross-section through a thin plate having thereon resonators which are separated from one another by stiffness diminutions; Fig. 5 is a cross-section through a thin plate having net-shaped stiffenings; Figs. 6a and 6b show a plate composed of two partial plates having spherical depressions; Figs. 7 to 9 show three different arrangements of plates in an air flow for generating an incoherent pressure impingement; Fig. 10 shows an arrangement wherein the plates are arranged in the fashion of a plate heat exchanger; and Fig. 11 shows the use of the sound-damping elements of the invention on wings or airfoils or supporting surfaces.

Fig. 1 shows, in a top view, a detail from a plate 10 having resonators 11, 12, 13 and 14, which together constitute a resonator collective body 1 7. For this, the resonators 11 to 14 are mutually separated from one another acoustically by stiffenings 1 6 and are framed by stiffenings 1 5. The latticed frame parts 1 6 have a lesser stiffness than the higher order frame parts 1 5.

The natural frequencies of the resonators 11 to 14 are therefore tuned to the upper frequency range of the sound waves that are to be damped, so that the natural frequency of the resonator collective body 17 is comparatively low. As a result of this tuning, the resonators and the resonator collective body are decoupled from one another and can oscillate independently of one another.

Because of the two-stage hierarchy of the frames 15 and 1 6, the surface of the entire plates is utilised doubly for the sound damping.

Shown in Fig. 1 also, in a top view, is a detail from a sound-damping plate 20 in which a resonator collective body 21 is subdivided threefold hierarchically. For this, the resonator collective body 21 framed by the higher order frame parts 22 is subdivided by a first subordinate or secondary frame structure 23 into subordinate resonator collective bodies 24. Each subordinate (or secondary or subsidiary) resonator collective body 24 is subdivided by a further subordinate frame structure 25 into individual resonators 26.

The natural frequency of the resonator collective body 21 should be smaller than that of the resonator collective bodies 24 and this in turn should be smaller than that of the individual resonators 26. This stipulation can be achieved by appropriate choice of the stiffness of the individual frame parts as well as by interruptions in the respective frames.

The rectangular frame structure shown in Figs.

1 and 2 can, of course, be replaced by other structures, for example by hexagonal structures.

Fig. 3 shows a cross-section through a plate in which circular resonators 31 are produced by two-sided sandblasting of the plate 30. As a result of the sandblasting, the worked surface parts becomes softer and thus of lower frequency.

The non-worked plate parts 32 therefore form the stiffer frame parts similarly to those in Figs. 1 and 2. By appropriate dimensioning of the worked and the unworked plate parts, in turn the previouslydescribed higher order latticed frame structures can be produced. The resultant resonators and resonator collective bodies are distributed frequency wise over the sound range to be damped.

Fig. 4 shows the cross-section through a plate 40 in which individual resonators 41 are separated from one another by corrugations 42.

With adequate stiffness of the resonators, these can oscillate in a cophasal manner and thus have a higher surface utilisation.

In the case of the exemplified embodiment shown in Fig. 5, the plate 50 consists of two partial plates 50.1 and 50.2, between which webs 52 forming a lattice are clamped. The partial plates 50.1 and 50.2 are pressed together and bonded to one another, so that the regions between the webs 52 can form individual resonators 51 which are capable of oscillating.

Through the dimensioning of the webs 52, again the higher order frame structures shown in Figs. 1 and 2 can be produced.

Fig. 6b shows a particularly effective exemplified embodiment of a sound-damping plate 60 which is produced from two partial plates 60.1 and 60.2. Each partial plate 60.1 and 60.2, has, at regular intervals, spherical impressions 61. With a wall thickness d of the partial plates 60.1 and 60.2, the height h of each spherical impression 61 preferably amounts to approximately 0.5 to 5 times the wall thickness d, whilst the diameter D of a depression is largely arbitrary. The partial plates 60.1 and 60.2 are so bonded together that the depressions 61 lie opposite either with the concave sides or, as in Fig. 6a, with the convex sides. The partial plates 60.1 and 60.2 are bonded together under pressure, so that they form a flat plate 60. The adhesive layer 62 also has damping properties.

Upon loading of the plate 60, the original depressions 61 have a non-linear spring characteristic, comparable to a cup spring. With an arch height h=d. 4, these have at a minimum the spring constant 0. For h > d. there arises an unstable region having a negative characteristic accordingly, with h < d. , any positive spring characteristic can be set. With the mass of the material of two original depressions 61 bonded together there arises a characteristic natural frequency. If a sound pressure is present on one side of the plate 60, then the regions F of the original depressions 61 oscillate in their respective natural frequency too and remove energy from the sound field. Through the dimensioning of the plate regions 63 remaining between the inward archings 61, again, similarly to in Figs. 1 and 2, higher order frame structures can be produced, so that the individual resonators are combined into resonator collective bodies.

Figs. 7 to 9 show various arrangements of plates of the aforedescribed kind as used in ventilating channels. In the case of such arrangements the problem exists of so constructing the inlet and/or the outlet of the ventilating channel that different sound pressures are present on each side of each plate (incoherent pressure impingement). With a coherent pressure impingement, the individual resonators and resonator collectives would not be excited.

In Fig. 7 the individual plates 71,72,73 are arranged parallel to one another and are of different lengths. In the case of the air inlet, indicated by the arrows shown, into the respectively formed channels 70, the partial channels formed by the individual plates have different impedance, so that also the penetrating sound fields are different.

In Fig. 8 the air inlet 80 between the plates 81 is so formed that two resonators units, each having different resonator covering occupancy, form a coulisse sound damper 82. These coulisse sound dampers act on sound waves in such a way that after a specific flow path single-layer plates having resonators and resonator collective bodies find a different sound field.

In Fig. 9 the air inlet 90 is formed by differently contrived or positioned plates 91 and 92. This provides for different sound pressure at the individual resonators and resonator collective bodies.

In Fig. 10, the plates 101 and 102, occupied with resonators and resonator collective bodies, of an air-channel sound damper are arranged one above the other in the fashion of a plate heatexchanger. The different pressure impingement of the individual plates 101 and 102 is achieved by locally offset air inlet apertures which are shown by arrows. For this, shutters 103 and 104 respectively arranged in an offset manner are disposed between the adjacent plate pairs 101 and 102.

Shown in Fig. 11 is an example of the practical use for the sound-damping elements in accordance with the invention consisting of resonators and resonator collective bodies. In this, the upper side of an aircraft wing 110 is produced, at least in the region of the trailing edge of the wing, from a plate having the resonators and resonator collective bodies. The high admittance of the resonators 111 damps pressure instabilities upon the flow therearound.

In this way, the flow can be kept stable for longer, so that a lesser resistance and a higher lift can be achieved.

Claims (8)

Claims

1. A sound-damping element having resonators consisting of a continuous thin plate connected in force-locking manner to a latticed frame and/or provided with net-shaped stiffenings or stiffness diminutions in such a way as to provide a plurality of resonators capable of oscillating independently, characterised in that the frame and/or the stiffenings or stiffness diminutions have different stiffnesses in one or more lattices or net structure of higher order, so as to provide resonator collectives having a resonance frequency which is lesser as compared with an individual resonator.

2. An element as claimed in claim 1, characterised in that the diameters of the resonator collectives are smaller than half the smallest wave length, to be damped, of the air sound.

3. An element as claimed in claim 1 or 2, characterised in that the individual resonators in the or each resonator collective have different resonance frequencies.

4. An element as claimed in claim 1, 2 or 3, characterised in that the resonators are acted upon with sound waves from one side and act with the other side on a closed or sealed-off gas volume.

5. An element as claimed in claim 1,2 or 3, characterised in that the resonators are acted upon on both sides by sound waves having a different phase relationship.

6. An element as claimed in any preceding claim characterised in that the plate forming the resonators comprises two partial plates which each have spherical impressions or indentations, the impressions of one of the partial plates lying in register with those of the other partial plate, with the convex or concave sides directed at one another and are bonded together under pressure in the region of elastic deformability into a plate without residual intermediate spaces.

7. An element as claimed in any preceding claim characterised in as a result of appropriate choice of material or of connection to a damping mass or composition, the resonators have a high inner inherent damping.

8. An element as claimed in any of claims 1 to 7 characterised in that the plates are arranged, in the fashion of a plate heat exchanger, with air inlets offset through 900.

9 A sound-damping element as claimed in any of claims 1 to 7, characterised in that the plates are arranged on the outside of the aircraft wing, an airfoil, or a supporting surface or lifting surface.

1 0. A sound damping element substantially as hereinbefore described with reference to and as illustrated in any of the figures of the accompanying drawings.